Method and means for measuring carrier lifetime in epitaxial films

ABSTRACT

To measure the lifetime of carriers in epitaxial film, contacts are made to the film by probes which are spaced from each other. A voltage is applied between the probes, and a light pulse is directed onto the film between the probes. The length of time that it takes the current flow through the layer between the probes to decay to a value of 1/e of the maximum value thereof after the end of the light pulse is a measure of the lifetime of the carriers in the layer.

United States Patent [191 Nikirk et a1.

[11] 3,745,454 [451 July 10,1973

[ METHOD AND MEANS FOR MEASURING CARRIER LIFETIME IN EPITAXIAL FILMS[75] Inventors: Roger G. Nikirk, Scottsdale; Leo C.

Rogers, Phoenix, both of Ariz.

Related US. Application Data [63] Continuation of Ser. No. 861,040,Sept. 25, 1969,

abandoned.

[52] US. Cl 324/64, 324/62 R, 324/158 D, 324/158 P [51] Int. Cl G0lr27/14 [58] Field of Search 324/62, 64, 158 D, 324/158 P; 331/945 [56]References Cited UNITED STATES PATENTS 2,677,106 4/1954 Haynes et a1.324/158 2,805,347 9/1957 Haynes et a1. 324/158 X 3,229,200 [I 1966Rayburn 3'24/62 3,312,893 4/1967 Currin et a1. 324/64 3,466,539 9/1967Pitts et al 324/62 3,495,170 Biard et a1. 324/62 OTHER PUBLICATIONSClarke et 211., [RE Standards On Measurement of Minority-CarrierLifetime, Proceedings of IRE, Vol. Y9, Aug. 1961, pp. 1292-1299,TK5700.17 Winogradoff, Radiative Recombination Lifetimes in LaserExcited Silicon, Applied Physics Letters, Vol. 8, No. 4, 2/15/66 pp.99-101 Tulk, Resistivity***Lifetime Measurements Semiconductor Products,Oct. 1960, Vol. 3, No. 10, TK 7800 S4, pp. 27-30.

Primary Examiner-Stanley T. Krawczewicz Attorney-Mueller & Aichele [5 7]ABSTRACT To measure the lifetime of carriers in epitaxial film, contactsare made to the film by probes which are spaced from each other. Avoltage is applied between the probes, and a light pulse is directedonto the film between the probes. The length of time that it takes thecurrent flow through the layer between the probes to decay to a value ofHe of the maximum value thereof after the end of the light pulse is ameasure of the lifetime of the carriers in the layer.

6 Claims, 4 Drawing, Figures PATENTED JUL 1 own Time INVENTOR. Roger G.M'k/r/r Leo C Rogers Arm sh METHOD AND MEANS FOR MEASURING CARRIERLIFETIME IN EPITAXIAL FILMS This .is a continuation, of application,Ser. No. 861,040, filed Sept. 25, 1969 now abandoned.

BACKGROUND As at present known, thereis no method or means for measuringthe lifetime of carriers in an epitaxial film. When measuring thecarrier lifetime in crystals, the crystal is prepared for measurement bycutting flat faces on each end thereof and a contact is provided thereonby applying a silver solution on the flat faces at the ends of thecrystals. Also the surface of the crystal away from the ends is vaporhoned to allow the light which is applied to the crystal to act on thecrystal in such a manner as to release carriers. The variation inapplying contacts to the ends of the crystal and in the vapor honing ofthe crystal may cause great variations in the resultsobtained inmeasuring the carrier lifetime. A variation of as much as 280 percent inlifetime measurements, which result from such variations, is acceptable.The current supply is allowed to find its own level at about 6 tomilliamperes, and a beam of light which has many wave lengths and whichhas relatively highdivergence is applied to the crystal to produce thecarriers. The fall time of the light pulse from the source may be to 40microseconds. Since the lifetime of the carriers in the crystal may belong compared to the fall .time of the light pulse, such a long falltime is not detrimental to the measurement of the lifetime of thecarriers in crystals. However, in measuring the lifetime of carriers inepitaxial layers, such a known method cannot be used since measurementsare not made between parallel ends of crystal and since the currentintensity used must not affect the epitaxial layer and since the carrierlifetime is so short that the fall time of a particular light may belonger than the lifetime of the carriers whereby the fall time makes themeasurement impossible using such a light. Furthermore, use of a beam ofwhite light, in which many wavelengthsof light are applied at the sametime produces readings dependent on other properties of the crystalwhich are a function of the various thousands of wave lengths in whitelight but not necessarily on the life time of the carriers in thecrystal.

It is an object of this invention to provide a method and a means tomeasure carrier lifetime in epitaxial layers.

SUMMARY In accordance with this invention, the contact ends ofspacedinsulated probes are applied to the epitaxial layer at a pressureintermediate a predetermined maximum and minimum pressure, a voltage isapplied between the probes having a value such that while enough currentflows through the epitaxial layer to provide repeatable measurements,the current flow does not injure the layer, and a laser device whichprovides high intensity pulses of light in a substantially nondivergentbeam and of a single wavelength, the pulses having a very short falltime, is directed on the area of the epitaxial layer between the probesto produce the carriers whose lifetime is to be measured. The flow ofcurrent through the layer is applied to an oscillograph which providesan indication of the current flow through the layer. The time it takesthe current to decay after a pulse has ended is plotted against time.The time that it takes the current to decay from its maximum to a valueequal to He of the maximum is the lifetime of the carriers.

DESCRIPTION taxial layer according to this invention,

FIGS. 2.,and 3 are curves useful in explaining the here disclosedinvention and FIG. 4 illustrates one type of contacts or probes that maybe used in the circuit of FIG. 1.

Turning first to FIG. 1, an epitaxial layer or film 10 has beendeposited in a known manner on a single crystal substrate 12, and thelifetime of the carrier in the film or layer 10 is to be measured. Apair of probes 14 and 16 having rounded contacting; tips are applied ata predetermined pressure and at spaced points thereon .to the layer 10.The predetermined contact pressure may be provided by a structure suchas that of FIG. 4 which will be described. The contact pressure must besufficiently high so that good contact is provided and yet not so highas to cause damage to the film 10 by the ends of the probes 14 and 16.Theprobes l4 and 16 are connected together through a series circuitwhich includes a voltage source such as a battery 18, an output resistor20 and a variable resistor 22 connected in series. The voltage of thebattery 18 and the values of the resistors 20 and 22 as well as thedistance between the contacts 14 and 16 is so chosen that the maximumcur rent flow through the film 10 is about one milliampere. However, thevalue, specifically, is optimized for each structure of epitaxual layerand substrate combination that is tested. The current flow must besufficiently high that good measurements result, and yet not so highthat the heat caused by the flow of current through the film 10 damagesthe film. A laser is directed on the surface of the film 10 in the areabetween the contacts 14 and 16, the laser beam being only slightlydivergent and comprising light of one wavelength such as aboutninetenths ofa micron long. The energy of this beam of this wavelengthwill be absorbed by the epitaxial layer 10. The beam has slightly moreenergy than that required to cause a transition of the electrons in thesilicon comprising the epitaxial layer to the conduction band from thevalence band. Shorter wavelengths of light will be reflected upon thesurface and longer wavelengths of light may not have sufficient energyto excite the electrons. The pulses of light produced by the laser 24has a fall time, that is the time that the light decays from its maximumvalue to a value equal to He of the maximum value, of one to twomicroseconds. An oscilloscope 26 is connected across the'output resistor20. The time that it takes for the trace 28 on the face 30 of theoscilloscope 26 togo from the top point of the trace 28 to a point'downthe trace where its value is equal to He times the peak value is ameasure of the lifetime of the carriers in theepitaxial layer 10..

In the operation of the circuit of FIG. 1, light pulses from the laser24, in reacting with the surface of the epitaxial layer 10 causeselectrons of the material of this layer 10 to be energized to the pointwhereby they become conductive electrons. The resistance of the circuitbetween the probes l4 and 16 decreases while the light from the laser 24illuminates the surface of the film and the top of the trace 28 showsthe current flow through the film. As noted above, the resistor 22 isadjusted so that the maximum current flowing through the film is aboutone milliampere. After the light produced by the lasser has ceased,current keeps flowing in the film 10 due to the fact that the conductiveelectrons produced by the light take time to throw off their energy anddescend into their valence band where conductivity of the film ceases. 1

However, the light at the end of a pulse does not end instantaneouslybut becomes less intense very rapidly at first to a point such as A ofFIG. 2 and then decreases in intensity at a lesser rate and becomes zeroat a much later time. Since the decrease is gradual after the point A isreached, the measurement is taken at the point where the current hasdecreased to a value of He of its maximum value, this point being at ornear the end of the rapid decrease in illumination as the pulseextinguishes.

As the light of the pulse, in the process of ceasing, decreases inintensity, the number of conductive electrons produced by the light beamdecreases in number also. After the light ceases entirely, no newconductive electrons are produced, however the conductive electronsproduced by the light takes time to descend to their valence stateduring which time they conduct, whereby the shape of the decreasingportion of the pulse 28 is caused both by the finite time it takes theapplied light to extinguish completely and also by the finite time thatit takes the conductive electrons to become valence electrons. FIG. 2 isa plot of the intensity of light of a known light source plotted againsttime. In FIG. 2, as the light produced extinguishes, light intensitydecreases fairly rapidly from its maximum to the point A and much slowerbeyond the point A. If the lifetime of the conductive electrons is shortwith respect to the decay time of the light to the point A, the lightsource having the light decay properties of FIG. 2 will present a curvesuch as 28 of FIG. 1 in which the time delay from the peak to the pointA is mainly due to the time it takes the light to extinguish and only toa very minor extent to the lifetime of the carriers, whereby when usinga light such as that which produces the curve of FIG. 2, the curve 28 ofFIG. 1 gives very inaccurate results if the lifetime of the carriersthat is to be measured is short. The curve of FIG. 3 is a plot of theintensity of light plotted against time for a laser such as 24 ofFIG. 1. In FIG. 3, the trailing end of the curve from the high pointthereof to the point A is very steep. Therefore the delay indicated bythe trailing edge of the curve 28 down to the point A when a laser 24 isused as a light source is to a great extent due to the lifetime of thecarriers and only to a minor extent due to the time it takes the lightwhich hits the epitaxial surface 10 between the probes l4 and 16 todecay to the point A of FIG. 3. The use of a laser in the method Theprobes 14 and 16 may be mounted as shown in FIG. 4 to assure that thepressure applied thereby to a film is that which is desired. In FIGS. 1and 4, the same reference characters have been given similar elements.

A body member 32 of insulating material has a pair of holes therein, 34and 36. The probes l4 and 16 are slideable in the holes 34 and 36respectively and they are urged, downwardly as viewed in FIG. 4 byrespective leaf springs 38 and 40. The probes l4 and 16 have respectivestops 42 and 44 projecting therefrom and cooperating with the slots 46and 48 in the body member 32 to prevent the probes l4 and 16 fromfalling out of the body portion 32. The springs 38 and 40 are held inplace in the body portion 32 by respective blocks 50 and 52 andrespective wires are connected to the springs 38 and 40 and therefor tothe probes l4 and 16 for connection in the circuit of FIG. 1. The topplate 54 of the body portion 32 may be fixed in place as by a screw 56.The bottom of the body portion 32 has pads 58 and 60 made of a softresilient material such as nylon fixed at the bottom thereof. The body32 is skele tonized as much as possible as shown in FIG. 4 to permitlight to hit the epitaxial area between the probes 14 and 16. Thesprings 38 and 40 are so weak that the weight of the body portion 32including the top 54 causes the lower ends of the pins 14 and 16 to beflush with the bottoms of the pads 56 and 58 when the body 32 is laid ona surface, whereby the pressure applied to the epitaxial layer 10 by theprobes 14 and 16 is determined not by the weight of the body portion 32plus the weight of the top 54 but by the stiffness of the springs 38 and40. Therefore, using a probe holder such as that of FIG. 4, the pressureapplied to the epitaxial layer 10 by the probes 14 and 16 can be set ata desired value, as by choice of the stiffness of the springs 38 and 40,and this pressure is reproducible from test to test.

What is claimed is:

l. The steps in the method of determining the lifetime of carriers in anepitaxial film which comprises:

pressing a pair of electrically conductive probes against the same sideof said film at predetermined spaced points thereof,

applying a predetermined voltage between said probes,

directing a pulse of light of one wavelength on said film in thevicinity of and between said probes, said wavelength being selected suchthat energy of said pulse of light is absorbed by said epitaxial film,and;

indicating the time duration of the flow of current between said probesafter the end of said pulse of light.

2. The invention of claim 1 in which the pressure which is applied tosaid film by said probes is sufficiently great to provide good contacttherebetween but without substantially distorting said film.

3. The invention of claim 1 in which the voltage which is appliedbetween said probes is chosen to be sufficiently great so that a goodindication is produced and yet not sufficiently great to damage saidfilm by heating.

4. The invention of claim 1 in which light pulses having a wavelength ofsubstantially nine-tenths micron from a laser are directed on saidepitaxial layer and said predetermined voltage causes maximum current ofsubstantially one milliampere to flow during said pulse of light.

ing a source of voltage and a current limiting means and an output meansin series between said probes, means for indicating the current flow insaid output means and, a light pulse producing laser for illuminatingthe portion of said film which is contacted by said probes. 6. Apparatusaccording to claim 5 wherein the weight of said body is greater thansaid predetermined force.

1. The steps in the method of determining the lifetime of carriers in anepitaxial film which comprises: pressing a pair of electricallyconductive probes against the same side of said film at predeterminedspaced points thereof, applying a predetermined voltage between saidprobes, directing a pulse of light of one wavelength on said film in thevicinity of and between said probes, said wavelength being selected suchthat energy of said pulse of light is absorbed by said epitaxial film,and; indicating the time duration of the flow of current between saidprobes after the end of said pulse of light.
 2. The invention of claim 1in which the pressure which is applied to said film by said probes issufficiently great to provide good contact therebetween but withoutsubstantially distorting said film.
 3. The invention of claim 1 in whichthe voltage which is applied between said probes is chosen to besufficiently great so that a good indication is produced and yet notsufficiently great to damage said film by heating.
 4. The invention ofclaim 1 in which light pulses having a wavelength of substantiallynine-tenths micron from a laser are directed on said epitaxial layer andsaid predetermined voltage causes maximum current of substantially onemilliampere to flow during said pulse of light.
 5. Apparatus formeasuring the lifetime of carriers in an epitaxial layer whichcomprises: a pair of conductive probes for contact with one side of saidepitaxial layer, a body having holes in which said probes are slideablymounted, spring means tending to slide at least an end portion of saidprobes out of said body with a predetermined force, a circuit connectingsaid probes, said circuit including a source of voltage and a currentlimiting means and an output means in series between said probes, meansfor indicating the current flow in said output means and, a light pulseproducing laser for illuminating the portion of said film which iscontacted by said probes.
 6. Apparatus according to claim 5 wherein theweight of said body is greater than said predetermined force.